Instructions (no sleeve version)

Print both pieces, one is the garden hose thread and socket, the
other is the locking sleeve.

Ream out the holes in the center part until the BB's can just barely
protrude through the side, but not so much that they risk falling
through. The locking sleeve can now be slid atop as an extremely tight
fit, becoming loose again (though nearly unremovable) once the balls
are fitting into the locking grooves.

Paint the very end of the threaded section with ABS glue, where it
would press against the hose washer, for the best seal.

Instructions (sleeve version)

Clean up the small sleeve, paint the inside of the threaded section
thinly with ABS glue, and force in the small sleeve. Wipe up excess
glue before it dries. The vertically-printed sleeve will have a
better seal than the horizontally-printed spindle.

Ream out the holes in the center part until the BB's can just barely
protrude through the side, but not so much that they risk falling
through. The locking sleeve can now be slid atop as an extremely tight
fit, becoming loose again (though nearly unremovable) once the balls
are fitting into the locking grooves.

Paint the very end of the threaded section with ABS glue, where it
would press against the hose washer, for the best seal.

Notes

The locking grooves hold the BB's fast when the sleeve is in the
locked position, preventing the other half of the quick-connect from
leaving. pushfit-002.jpg shows it in locked position, with the sleeve
down.

Like most garden hose things, use proper hose washers to prevent leaks.